1. Field of the Invention
This invention relates to hybrid type rock bits.
More particularly, this invention relates to a multi-cone rock bit with drag bit type diamond cutters positioned in the face of a drag bit portion of the bit and the means in which the innermost diamond disks are attached to the face of the drag bit portion.
2. Description of the Prior Art
Hybrid bits of the type that combine multi-cone cutters with drag bit cutters are known in the art.
U.S. Pat. No. 4,006,788, assigned to the same assignee as the present invention, describes a rock bit for recovering core samples as well as rock bit variations for drilling oil wells or the like. In each of the several embodiments described, diamond cutters are strategically mounted on a bit body for cutting rock by a shearing action. Each of the diamond cutters is in the form of a thin diamond disk bonded to a tungsten carbide stud that is inserted into the bit body. The diamond inserts are generally mounted in the drag bit portion of the hybrid bits. The inserts are typically mounted by interference fitting the body of the diamond insert into the face of the drag bit portion. The face of the drag bit portion is drilled, followed by insertion of the diamond insert body into the drilled hole.
This patent is disadvantaged in that the innermost diamond cutting insert blanks nearest the center of the borehole cannot be interference mounted in the face of the drag bit portion. The inserts cannot be mounted close enough to the center of the hole to remove the core of the borehole. This is true because of the necessity of shaping the drag bit portion into a wedge or pie-shaped configuration. The tip of the drag bit face portion nearest the center of the bit provides the least area to mount the diamond insert into the body of the drag bit portion.
The present invention describes a means in which the innermost diamond cutting disk may be mounted to the tip of the pie-shaped drag bit face, thus providing a means to remove the core portion in the area of the center of the borehole bottom. The central area of the borehole bottom is one of the most difficult areas from which to remove material.